1. Field of the Invention
This invention relates to sound synthesis and more specifically to a method and apparatus for preventing sound artifacts (pops and clicks and "zipper" noise) when a sound parameter such as volume level is changed.
2. Description of the Prior Art
In sound synthesis it is well known that when a sound parameter, such as volume changes, undesirable sound artifacts such as clicks or pops occur. These are wideband sound artifacts caused by an abrupt transition in the sound parameter. These artifacts are also called control noise. FIG. 1A shows graphically a sound waveform 10 inside a volume envelope 12 experiencing an abrupt transition, having corners 14, 16. The horizontal axis is time and the vertical axis is sound volume. The artifacts occur at the time of the transition (at corners 14, 16).
Prior art methods to eliminate these artifacts include linear and exponential ramping as shown respectively in FIGS. 1A and 1B. In FIG. 1B a linear ramp 18 is provided between the higher sound level at time t.sub.0 20 and the lower sound level at time t.sub.1 26. This is instead of an abrupt vertical drop as in FIG. 1A which causes the undesirable clicks or pops. A better solution is shown in FIG. 1C where at the lower end of the ramp 22, an exponential decay 24 is provided (an exponential ramp). However, the examples of FIGS. 1B and 1C, even though they cause less of a problem than the vertical change of FIG. 1A, still have the problem that at the corners 20, 26 in FIG. 1B and the single corner 28 in FIG. 1C, the derivative of the ramp function at the corner is discontinuous causing a click or pop. Thus these prior art solutions still have sharp, non-differentiable transitions at the corners and thus are not complete solutions.
Another solution is to provide an extended muting period i.e., stretch out the length of the ramp over a much greater time, thus further reducing the sharpness of the corners. However, this still does not completely eliminate the corners and requires a longer time, thus preventing a rapid change in the sound parameter. The muting approach may take as long as 1/10th of a second using an exponential or linear ramp and still not provide complete elimination of the clicking or popping artifacts. Also, the prior art approaches are not capable of dealing with a dynamically changing parameter where the ramp endpoint changes in the middle of ramp, as shown in FIG. 1D, causing a discontinuity at corner 30 in the volume envelope 31, due to a later upward change in target volume level during an earlier downward volume ramp.